Ultrafast Excited State Dynamics and Fluorescence from Vitamin B₁₂ and Organometallic [Co]–C≡C–R Cobalamins

Abstract

Cobalamins are cobalt-centered cyclic tetrapyrrole ring-based molecules that provide cofactors for exceptional biological processes and possess unique and synthetically tunable photochemistry. Typical cobalamins are characterized by a visible absorption spectrum consisting of peaks labeled α, β, and sh. The physical basis of these peaks as having electronic origin or as a vibronic progression is ambiguous despite much investigation. Here, for the first time, cobalamin fluorescence is identified in several derivatives. The fluorescence lifetime is ca. 100–200 fs with quantum yields on the order of 10 –6 –10 –5 because of rapid population of “dark” excited states. The results are compared with the fluorescent analogue with zinc replacing the cobalt in the corrin ring. Analysis of the breadth of the emission spectrum provides evidence that a vibrational progression in a single excited electronic state makes the dominant contribution to the visible absorption band.

Vibronic Coupling in Vitamin B12: A Combined Spectroscopic and Computational Study

“…For example, triply resonant sum frequency (TRSF) spectroscopy, a multidimensional spectroscopic technique capable of identifying vibronic coupling, yielded results consistent with a vibrational progression, although the authors were unable to definitively distinguish between the two models . In another recent study, ultrafast transient Abs spectroscopy was used to demonstrate that the bulk of the Abs intensity in the α/β region is due to a vibrational progression associated with a single electronic transition, with the possibility of a minor contribution from another electronic transition …”

“…58 In another recent study, ultrafast transient Abs spectroscopy was used to demonstrate that the bulk of the Abs intensity in the α/β region is due to a vibrational progression associated with a single electronic transition, with the possibility of a minor contribution from another electronic transition. 59 Although these experimental data favor the existence of a vibrational progression in the α/β region, previous computational investigations of this region were complicated by the fact that the choice of functional greatly impacts the description of TD-DFT predicted excited states. Consistent with the extensive literature on this subject, the pure GGA functional BP86 predicts three electronic transitions of significant intensity in the α/β region, all of which can be characterized as d/π → π* (MLCT) and π → π* excitations (Figure 3).…”

Deciphering Photoinduced Catalytic Reaction Mechanisms in Natural and Artificial Photosynthetic Systems on Multiple Temporal and Spatial Scales Using X-ray Probes

“…Outgoing signals at each XFEL shot fluctuate significantly because of the stochastic nature of the SASE XFEL pulses, with complex spectral and temporal pulse profiles and significant shot-to-shot spectral, temporal, and intensity fluctuations, along with the volume fluctuations in sample delivery.Therefore, new methods are required for data normalization compared to what is traditionally used at synchrotron sources (i.e., normalization by the incident X-ray intensity, I 0 ). Despite these challenges, collecting XAS data at XFELs is possible with continuous jet and drop-on-demand sample delivery methods. − Scattering signals from the sample, collected at the downstream 2D detector, allow normalizing XAS signals, as they are linearly related to the probed sample volume and intensity of the X-ray pulses for each shot. , Efforts toward further reducing sample concentration (<1 mM) and collecting data within a reasonable time are underway …”

Antivitamins B12—Some Inaugural Milestones

“…[67] This work has led to fruitful research collaborations, discovering new organometallic Cbl-chemistry,p hotochemistry and biochemistry. [22,24,68] It has, likewise, opened up new avenues in the field of the fascinating transition metal analogues of the Cbls and of other natural corrinoids. [34,36,39,42] The helical, ringcontracted natural corrinl igand has been characterized as an exceptional 'Procrustean Bed' for bound transition metal ions, important for tightly binding and specifically activating the bound cobalt-ions in their low-spins tates.…”